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The NASA Hubble Space Telescope Science Release HEIC 1320 - When is a Comet not a Comet? - Hubble astronomers observe bizarre six-tailed asteroid is self-explanatory, but it doesn't explain what actually happening.

Sometimes comets will show two tails, but one is usually identified with the path of the comet's orbit, and the other with the direction away from the sun. But this is an asteroid (or so it seems) and there are many tails with complex behavior.

edit: I've found this ArXiv preprint: The Extraordinary Multi-Tailed Main-Belt Comet P/2013 P5 where "main belt" refers to the main belt of asteroids, and yet the object is now called a comet. Wikipedia lists both names and suggests both designations - asteroid or comet:

311P/PANSTARRS also known as P/2013 P5 (PANSTARRS) is an asteroid (or main-belt comet) discovered by the Pan-STARRS telescope on 27 August 2013.(4) Observations made by the Hubble Space Telescope revealed that it had six comet-like tails.(5) The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile asteroid spinning fast enough to remove material from it.(2)

It's been three point five years now - has this been figured out yet? Is it an asteroid or comet (or both), and how can it produce so many tails?

enter image description here above: HST image of comet P/2013 P5 from here.

enter image description here above: HST image (annotated) of comet P/2013 P5 from here.

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    $\begingroup$ Have you tried contacting D. Jewitt directly? his email is on that page. $\endgroup$ – Carl Witthoft Nov 14 '16 at 13:39
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    $\begingroup$ @CarlWitthoft if I can find an answer here, then everyone can benefit. I don't think Astonomy SE should be thought of only as a last resort. $\endgroup$ – uhoh Nov 14 '16 at 13:46
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    $\begingroup$ You've got it backwards: go find the information, then post it as an answer yourself. $\endgroup$ – Carl Witthoft Nov 14 '16 at 18:47
  • $\begingroup$ @CarlWitthoft I'm shy... $\endgroup$ – uhoh Feb 6 '17 at 15:58
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    $\begingroup$ I wonder if multiple and changing tails could be caused by local surface events. The thin crust is broken up at one place and a jet of underground volatiles is released, eventually even turning the comet around causing further surface changes in the new sunlight conditions. $\endgroup$ – LocalFluff Feb 6 '17 at 19:21
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A paper titled "Monte Carlo Models of the dust environment of a sample of comets from the Oort Cloud to the outer Main Asteroid Belt" (PDF also here) (2014) studied the behaviour of P/2013 P5 using models and concluded that

  1. The object has been subjected to an intermittent dust mass loss, most likely associated with a rotational disruption. This is confirmed from the analysis of both HST and GTC images. The total dust mass released was of the order of $10^7\ kg$, for particle density of $1000\ kg\ m^{−3}$ and geometric albedo $p_v = 0.04$.
  2. The model of rotational disruption, based on simulations of an object that loses mass from its equatorial region, and whose rotational axis is perpendicular to its orbit plane, reproduces to the last detail the observed complex brightness pattern at four different epochs of HST and GTC observations. For obliquities different from 0° or 180° , the fits get much worse. On the other hand, an isotropic ejection model does not fit the HST data because it produces much more diffuse tails than observed.
  3. The ejection velocities are very low, of the order of $0.02–0.05\ m s^{−1}$. This places the limit on the size of the object to be in the range $30–134\ m$ for assumed densities of $3000–1000\ kg m^{−3}$.

This seems to confirm what the original question cited as a possible explanation, i. e.

The tails are suspected to be streams of material ejected by the asteroid as a result of a rubble pile asteroid spinning fast enough to remove material from it.

The asteroid is loosely bound (a "rubble pile") and spinning fast, so everytime some of the boulders which form it shift a bit, some material may escape into space and form a small tail.

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  • $\begingroup$ Hey great! Thank you for this. As far as the "asteroid or comet" part of the question, does the paper choose to use "asteroid" simply to avoid repeating "asteroid or comet" at every instance, or is it being actively asserted that the object is both not a comet and is an asteroid? $\endgroup$ – uhoh Feb 7 '17 at 3:02
  • $\begingroup$ Actually comet and asteroid are blurry categories. I guess the authors began with one term and stuck to it for convenience. There's a relevant paragraph at Wikipedia (Asteroid#Terminology) - "The main difference between an asteroid and a comet is that a comet shows a coma due to sublimation of near surface ices by solar radiation..." etc. $\endgroup$ – pablodf76 Feb 7 '17 at 10:17
  • $\begingroup$ Wow! Will this be in tomorrows news then? "Astronomers Decide Comets and Asteroids are the Same Thing - Now Considering Lumping Planets and Dwarf Planets Back Together Again" $\endgroup$ – uhoh Feb 7 '17 at 10:30
  • $\begingroup$ I've noticed in my first comment I should have said "...does the paper choose to use "comet" simply to avoid repeating "asteroid or comet" at every instance..." - there is a great answer here that addresses this subject further. $\endgroup$ – uhoh Feb 8 '17 at 2:21

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